1. Field of the Invention
The present invention provides a method and apparatus for removing ethylbenzene from a para-xylene feed stream. More particularly the method includes the pretreatment step of processing the feed stream through a reactor comprising a small version of an isomerization unit within which an isomerization catalyst is provided, the catalyst causing a high level of conversion of ethylbenzene to benzene without producing side reactions, such as cracking of xylenes. The benzene is then removed from the feed stream prior to cyclic para-xylene separation and isomerization, respectively. Although the apparatus and method are proposed for use in para-xylene production, they are equally well suited to use in production of ortho-xylene and meta-xylene.
2. Description of the Prior Art
Para-xylene is a precursor in the manufacture of polyesters, which are used in creating clothing and other synthetic materials. Most para-xylene is produced in oil refineries downstream of catalytic reformers which manufacture gasoline.
Gasoline contains a mixture of hydrocarbons including C8 aromatics. The C8 aromatics include four chemical compounds: para-xylene, meta-xylene, ortho-xylene and ethylbenzene. The para-xylene is isolated from the other C8 aromatics by one of two physical separation processes, crystallization or molecular sieve technology. Once the para-xylene has been removed from the mixture of C8 aromatics, the remaining mixture of meta-xylene, ortho-xylene and ethylbenzene is sent to an isomerization unit where further para-xylene is created by returning the xylene mixture to an equilibrium concentration, with the new ratio of xylene isomers being returned to the separation unit for reprocessing.
The concentration of para-xylene in the recycled feed to the separation unit determines the efficiency of the operation. For example, in a crystallizer, the mother liquor contains about 10% para-xylene. Therefore, if the recycled feed stream contains 19%, about 9% will be extracted and 10% will not be extracted. However, if the recycled feed stream contains 21% para-xylene, the production rate increases substantially due to an 11% extraction. Thus, increasing the para-xylene in the recycled feed stream from 19% to 21% will increase the plant capacity by approximately 11/9, or 20%.
In modern units, the ethylbenzene is dealkylated to benzene in the isomerization unit. This reaction proceeds at 50 to 60% conversion per pass. Thus, the recycled feed stream provided to the separation unit always contains a substantial amount of ethylbenzene. This ethylbenzene builds up in the mother liquor/recycled feed stream combination causing processing equipment to be larger than necessary to merely process the para-xylene, ortho-xylene and meta-xylene. Since the recycled feed stream to the separation unit contains approximately 20% para-xylene, the mother liquor will contain the unextracted para-xylene plus the remainder of the chemicals which are not para-xylene. Thus, the mother liquor/recycled feed stream becomes quite large, requiring large processing equipment. For a crystallizer, the mother liquor recycle is six times para-xylene flow rate. In a molecular sieve extraction unit, the recycle is about four times the para-xylene flow rate. If ethylbenzene were removed from the feed in an initial pretreatment step, the remaining stream would only have para-xylene, meta-xylene and ortho-xylene therein. Parameters of conditions required for an isomerization reaction to the point of equilibrium in the conversion of the meta-xylene and ortho-xylene to a combination thereof with created para-xylene become less severe if little or no ethylbenzene is fed to the isomerization unit. The severity is reduced because the point of equilibrium now revolves around only three xylenes, with no ethylbenzene contaminant needing to be considered. Also, the mother liquor flow rate per unit of para-xylene production is reduced, saving energy required for processing.
Accordingly, removing substantially all of the ethylbenzene from the feed stream would provide a considerable advantage inasmuch as the production of para-xylene would increase proportionally with the concentration of the para-xylene in the recycled feed stream and the isomerization unit would be more efficient in processing due to lack of ethylbenzene in the feed stream thereto.
Such removal of ethylbenzene from the feed stream is accomplished by the method and apparatus of the present invention.